An HARQ is a combination of an Automatic Repeat Request (ARQ) and a Forward Error Correction (FEC), and is a means of LTE system link adaptation.
An LTE system adopts a stop-and-wait HARQ protocol of N channels, that is, N processes exist simultaneously, and each process adopts a stop-and-wait ARQ protocol for transmission. After sending a data packet, a transmitting end stops temporarily to wait for an Acknowledgement (ACK) message of a receiving end; when data reaches the receiving end, the reaching end checks the data; and if the received data is correct, the receiving end feeds back an ACK message to the transmitting end; or else, the receiving end feeds back a Negative Acknowledgement (NACK) message to the transmitting end. The transmitting end sends new data after receiving an ACK signal, otherwise the transmitting end retransmits the last data packet. The parallel N processes are in the stop-and-wait process, and other processes may use channel resources for transmission.
The minimum Round Trip Time (RTT) of the HARQ is defined as the completion time of a data packet transmission process, including a process in which the data packet is sent; the receiving end receives and processes the data packet, and then feeds back the ACK/NACK message; and after receiving and demodulating the ACK/NACK signal, the transmitting end determines to retransmit data or send a new data packet. For a Frequency Division Duplex (FDD) frame structure, uplink and downlink transmissions are always continuous, and ACK/NACK signal feedback or data retransmission may be performed in a fixed subframe. For a Time Division Duplex (TDD) frame structure, since the uplink and downlink transmissions are in time division multiplexing, it is impossible to find for each subframe feedback time intervals which are fixed and the same. For different uplink/downlink configurations of TDD and different subframes, the time intervals of ACK/NACK feedback and retransmission are different.
With the development of communication services, a service requirement of lower latency requires a communication system to support a data transmission of lower delay, and a shorter Transmission Time Interval (TTI) becomes a major way of implementing a low-delay transmission. So, it is urgently needed to redefine an HARQ feedback timing for a downlink Physical Downlink Shared Channel (PDSCH) transmission or a Physical Downlink Control Channel (PDCCH)/Enhanced Physical Downlink Control Channel (EPDCCH) indicating a Semi-Persistent Scheduling (SPS) release in the case of a variety of shorter TTIs, so that an HARQ feedback time association in the case of the shorter TTIs may be determined.